1. Field
The invention is in the field of optical detection of lines on the surface of a highway from a moving motor vehicle.
2. State of the Art
Statistics from the National Highway Traffic Safety Administration show that vehicle lateral drift, i.e., drift from one lane to another or drift off the roadway, accounts for about 25% of fatalities on our nation""s highways. As a result, President Clinton has mandated motor vehicle safety devices be developed to help solve the problem and President Bush has continued the mandate.
It has been realized since at least 1973, U.S. Pat. No. 3,708,668, that it would be desirable to have a device for motor vehicles that would detect highway lines and sound or display an alarm to alert the driver of the vehicle when the vehicle approached or crossed such a line so the driver could correct and stay within his or her lane and avoid unintentional lateral drift. The search for a practical device to provide satisfactory highway line detection has continued and a number of devices have been suggested.
It was recognized as early as U.S. Pat. No. 3,708,668 that a significant problem with such a system is compensating for different ambient light levels. Thus, such a system has to be able to detect road lines in bright sunlight as well as in darkness at night. U.S. Pat. No. 3,708,668 provides a pair of sensors that balance each other when both detect a road surface, but result in a warning signal when one sensor detects a light reflecting line and the other does not. U.S. Pat. No. 4,143,264 provides a bridge circuit with an impedance element controlled by an integrated output of the bridge circuit to maintain a balance of the bridge for ambient light conditions and vary sensitivity of the detector with light conditions. Various infrared, laser, and CCD camera devices have also been suggested, U.S. Pat. Nos. 4,348,652, 5,979,581, 5,790,403, and 5,957,983. U.S. Pat. No. 5,982,278 shows various arrangements of detectors for detecting road lines and special line arrangements but does not teach any specific detection circuitry.
However, the need remains for a reliable line detector that can be built into a vehicle or provided as an aftermarket product to be easily mounted on a vehicle such as in or on side mirrors or mounted on one or more side windows of the vehicle.
As indicated, a major difficulty in the optical detection of highway lines is that the ambient light varies in intensity over many orders of magnitude from a sunny day to night. The detector must be able to accommodate these extremes, in particular during daytime, without current saturation. The inventor has discovered that the dynamic impedance characteristics of a reverse biased zener diode can be used to compress the voltage output signal of a phototransistor to accommodate extremes of light intensity. This approach is a simple and economical solution to the problem.
Dynamic impedance is, in general, defined as the incremental change in voltage across a device with respect to an incremental change in the current through the device.
The voltage at which the voltage across the reverse biased zener diode nearly ceases to increase with increases in current is referred to as the zener voltage. Zener diodes with various zener voltages are commercially available and should be chosen so that this voltage, or the series combination of these voltages, is below the saturation point of the photodetector in bright light, but so that the highest expected voltage across the zener diode in the series combination should always be less than the zener diode""s or series combination of zener diode""s zener voltage.